Digital, solid-state, variable-frequency drives (VFDs) can be powerful tools in maintaining processes by using diagnostics to solve drive performance issues and troubleshoot related processes. An understanding of how the VFD interacts with the process can help you improve overall production and product quality (Fig. 1).
VFDs are not infallible; sometimes they need to be repaired or replaced. The VFD is often the first indicator of a process change or application problem.
Many VFDs communicate using an LCD or LED display, or through an open interlock or fault indication. In most applications, the VFD interacts with operator controls, process control signals, and PLCs. A problem with the interaction between the VFD and these external controls may appear to be a drive issue, when actually the problem is with the process. Discussing process and drive symptoms with the machine operators often can help determine the problem area (see sidebar, “Talking with machine operators”).
If the external controls are working correctly, use the VFD to identify problems systematically. If the display status indicator does not operate, verify incoming ac power. If the status indicator still does not display after verifying or restoring ac power, then verify control power, and restore it if necessary.
If the VFD has been operating successfully, but suddenly fails to start, or if the drive starts but does not run properly, check to see if the diagnostics status display indicates a fault. The instruction manual for the VFD should have a description of faults and troubleshooting steps. Use diagnostics or a keypad control to monitor variables such as incoming voltage, dc bus, carrier frequency, output frequency, voltage, current, and I/O and control status. These parameters are displayed on most common VFDs. I/O status uses bits to monitor required start conditions to ensure they are enabled and to determine what may be inhibiting start. Control status indicates the source of the speed reference and can be used to verify incoming speed or direction signals.
High bus fault
High bus is a common fault caused by external factors. An instantaneous voltage spike in the ac line or an “overhauling load” created by the inertia of the machine can cause a high bus fault. The load continues to rotate faster than the motor’s commanded speed. When this situation occurs, the VFD protects itself by tripping on a high bus fault and shutting off the insulated gate bipolar transistors (IGBTs).
If a high bus fault is indicated, ensure that the ac power supply is consistent and that the deceleration time is adjusted to match the capability of the load. If the process requires rapid deceleration, dynamic braking or a regenerative power control circuit may be added (see sidebar, “Dynamic braking and regenerative power”).
Another common fault is overcurrent. When troubleshooting overcurrent faults, first check all power connections to ensure that they are properly attached. Loose connections or broken conductors frequently are culprits when overcurrent and control problems occur. Loose power connections cause overvoltage and overcurrent conditions, blown fuses, and VFD damage. Loose control wiring causes erratic drive performance, resulting in unpredictable speed fluctuations or the inability to control the VFD.
Use an autotuning feature if it is offered on the VFD. The autotuning function on many drives enables the drive to identify the attached motor, allowing rotor information to be used in the processor algorithms for more accurate current control. The VFD also can compensate for flux current, allowing better control of the torque-producing current. Both over and under fluxing the motor can be troublesome.
The second step is to check the mechanical load for worn or broken parts, or excessive friction. Repair or replace components as needed.
Finally, check incoming voltage and acceleration rate. If incoming voltage is too low, or the acceleration rate is set too fast, an overcurrent fault is possible. Decrease the acceleration rate or stabilize incoming voltage to correct this fault.
High starting-load current
High current/load readings may indicate mechanical binding or unexplained changes in process speed or load. The power requirements for many pumps and fans increase proportional to the cube of the rotational speed (S3). Running loads just a few revolutions per minute faster can overload a VFD.
Components should be checked before startup to avoid an overload situation. Conveyors left loaded during off hours should be unloaded before startup. Clogged pumps should be avoided by cleaning out solids that have settled while the pump was not in use. Avoid ice or moisture that possibly could form on the load. Wet material is heavier than dry and can place more loading on the conveyor, causing motor and VFD overload.
One way to reduce a high starting load is to use a VFD with an extended acceleration rate. This feature starts a load slowly and smoothly rather than jerking it to a start. This type of start is easier on mechanical components and has lower line requirements because the VFD draws only 100%%%MDASSML%%150% of load.
If the VFD is functioning erratically, but a fault is not indicated, external factors may be the cause, or the drive itself may have failed. Understanding the causes of VFD faults helps you determine the root cause of the problem. Frequently overlooked root causes are usually instabilities in the process that force the VFD to function in harsh conditions.
Visually inspect the VFD for burned or overheated components by looking for signs of discoloration or cracking. Burned or cracked components prevent proper VFD operation. Replace defective components and test the VFD before returning it to operation.
Power quality is another electrical issue that can affect a VFD. A change in utility equipment or unexpected power surges, due to electrical storms or system overloads, can affect VFD performance.
Contamination is a preventable cause of VFD failure. Check the VFD for contamination of dust, moisture, or other airborne particles that may be electrically conductive. Tracking or arcing marks across components or circuit board traces indicate evidence of contamination failures. If contamination is excessive, the VFD must be isolated from the contamination source by changing the environment or providing an appropriate NEMA-rated enclosure. If there is significant airborne contamination from dust, moisture, or corrosive vapors, the VFD must be in at least a NEMA-12 enclosure.
The internal cooling fans and component heatsinks of the VFD also should be checked for contamination. Blocked fans force the VFD to operate outside of its temperature specification, which can cause premature failure as a result of inadequate cooling. Check the fan for grease and other contaminants that can cause bearings and other parts of the fan to fail. Both the interior and exterior of the VFD, including fans, blowers, filters, and heatsink fins, should be cleaned monthly to reduce the risk of failure from contaminants.
The environment within which the drive must operate must be within specified temperature limits. Measure the temperature inside and outside the enclosure to ensure that it is within the ambient specifications determined by the manufacturer. Failure to meet the required temperature specifications can lead to premature VFD failure because numerous power components rely on adequate cooling for proper operation.
If the ambient temperature is too high, additional cooling should be added to the enclosure or the VFD should be relocated to an area where the ambient temperature is within the specification. Low ambient temperatures may cause problems as well. Condensation may form and cause component or VFD failure.
Many faults are caused by misapplication of the VFD. Process changes, such as variations in load or speed; power issues, such as capacity switching by the utility; or changes in environmental operating conditions are not immediately obvious, but could be a major contributor to VFD failure. Evaluate the consistency and condition of the process when trying to determine the cause of failure.
If the VFD remains inoperative after performing the aforementioned checks, contact the manufacturer. Most VFD suppliers have highly trained technical support personnel that can provide the assistance needed to diagnose the problem. The technical support staff can help you select replacement parts or a new drive if replacement is necessary.
As intelligent devices embedded in the manufacturing process, VFDs can provide insight into application and equipment performance. By providing the maintenance worker with the information necessary to understand and interpret the problem, VFD issues, and sometimes process or operational problems, can be quickly identified so that plant operation can be resumed and productivity improved.
If you have questions on VFD troubleshooting, contact the authors. Doug Palinkas can be reached at email@example.com ; Bryan Sisler can be reached at firstname.lastname@example.org . Both authors can be reached by phone at 864-284-5350. Article edited by Jack Smith, Senior Editor, 630-288-8783, email@example.com .
Talking with machine operators
Talking with machine operators can often identify the problem area. Useful questions to ask are:
What was happening with the machine at the time of failure?
Did the machine jam?
Did other devices trip at the same time as the drive?
Was there an electrical shutdown caused by lighting storm or brown-out?
Was there construction such as welding going on around the drive?
What was happening with the utility?
Are there power-factor-correction capacitors in the plant? If so, when are they switched?
Did the utility observe any disturbance?
Dynamic braking and regenerative power
When a process requires a drive and motor to decelerate rapidly, the motor can actually operate as a generator. The energy stored within the motor in the form of mechanical rotation must go somewhere. A dynamic braking circuit is used to accommodate this energy.
A dynamic braking circuit is a switch that monitors the dc bus and turns on when the bus level exceeds a certain setpoint. Energy is delivered to the resistors, and is expended in heat until the bus level drops below that setpoint.
A regenerative power supply operates as an inverter-in-reverse, which allows synchronization of the IGBT firing circuits with the incoming ac line. Using a regenerative power supply allows the circuit, which is separate from and external to the drive, to send the excess energy from the motor through the dc bus back to the power source.
VFD troubleshooting checklist
This table describes VFD faults, lists possible causes, and provides corrective actions. After reading the article, refer to this checklist when troubleshooting drives and related process problems.
|Fault||Possible Causes||Corrective Actions|
|High bus fault||Overhauling load||Adjust deceleration time to match load capability|
|Input voltage level too high||Install isolation transformer, line filtering|
|Instantaneous voltage spike||Ensure ac power supply is consistent|
|Overcurrent||Loose power connections or control wiring||Check all power and control connections and tighten loose connections or wiring|
|Incoming voltage too low||Stabilize/raise incoming voltage|
|Acceleration rate set too high||Decrease acceleration rate/lengthen acceleration time|
|Damage or excessive wear on mechanical load||Check load for damage/repair damage|
|Possible misalignments||Check and verify installation|
|Faulty motor||Repair or replace motor|
|Incorrect drive set up||Verify/adjust parameter settings|
|Drive/motor not matched to load||Verify application requirements|
|Overtemp||Clogged filter||Clean, repair, or replace filter|
|Change in ambient temperature||Provide for additional cooling/ventilation|
|Fan blade damage or fan loss||Repair or replace fan|
|No power on drive||Trouble with input ac power||Verify input ac power is connected and replace any blown fuses|
|Trouble with input control power||Verify input control power is connected %%POINT%% Check external conditions|